1. Field of the Invention
This invention, in its preferred form, relates to computer control of a plurality of lasing steps on a work piece and more particularly to such computer control for using a laser beam to carry out a sequence of machining steps on a work piece, wherein the parameters of generating the laser beam are automatically controlled as to the desired step and its mode. More particularly, this invention relates to computer control for laser machining that is calibrated to vary the power level of the generated laser beam in accordance with the desired mode, the set of laser parameters, and as a function of the measured power of the laser beam as directed onto the work piece. In an illustrative embodiment of this invention, the calibrated, computer control effects a series of welds on a work piece in the form of a nuclear fuel rod assembly made of a volatile metallic material such as the zirconium alloy known as Zircaloy.
2. Description of the Prior Art
The precision laser welding apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assembly 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser welding apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grid straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grid straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 48 and tab portions 50a and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
U.S. Pat. No. 3,966,550 of Foulds et al., and U.S. Pat. No. 3,791,466 of Patterson et al., assigned to the assignee of this invention, disclose similarly configured fuel rod grids of the prior art. Each of these patents discloses a fuel rod grid, wherein the inner and outer grid straps are made of a suitable metallic alloy such as Inconel, and the above identified interconnections are effected by furnace brazing. However, the zirconium alloy Zircaloy is known to have the desirable characteristic of a low neutron absorption cross section which allows for more efficient use of the nuclear fuel in the utility operation and therefore allows for a longer elapsed time between refueling by the replacement of the nuclear fuel bundle assemblies. In particular, fuel rod grids made of Zircaloy have a lower absorption rate of the neutrons generated by the fuel rods than that absorption rate of straps made with Inconel. The making of the grid straps of Zircaloy requires at least several changes in the assembly of the fuel rod grids. First, it is necessary to make the slots, whereby the inner grid straps may intersect with each other, of looser tolerances in that grid straps made of Zircaloy do not permit a force fitting thereof, i.e. to be hammered into position, but rather require controlled fit-up to allow "push-fits" of the intersecting grid straps. In addition, Zircaloy grid straps may not be brazed in that heating Zircaloy to a temperature sufficient to melt the brazing alloy would anneal the Zircaloy, resulting in a loss of mechanical strength.
Prior to the selection of a particular method of welding, several different methods of welding volatile materials such as Zircaloy were investigated including continuous welding with a CO.sub.2 laser, pulsed emission of a Nd: YAG laser, gas tungsten arc welding and electron beam welding. A pulsed electron beam is capable of power densities of up to 10.sup.9 watts/square centimeter with pulse widths in the micro-second and low milli-second range. However, welding with an electron beam is typically carried out in a vacuum environment which is relatively expensive to build and requires a relatively long time to establish the desired degree of vacuum therein, thus slowing down the manufacture of the fuel rod grids. Further, it is necessary to obtain relative movement of the work piece, e.g. the fuel rod grids, in three dimensions with respect to the electron beam which would require a very complex grid positioning system. The use of a continuous electron beam provides relatively low levels of power (in the order of 200 watts) requiring relatively long welding times and providing very shallow weld penetrations. The use of a gas tungsten arc was also considered and proved to be unacceptable for providing a sequence of welds in that after a given number of welds, e.g. 25, the arc electrodes require sharpening to provide the desired fine arc to produce numerous well-defined welds and to avoid damaging adjacent grid straps or vanes of the fuel rod grids. Two types of lasers are commonly used for welding applications: (1) the solid state Nd:YAG laser, which uses a crystal rod of neodynium doped yttrium-aluminum-garnet and (2) the CO.sub.2 laser, which uses a mixture of CO.sup.2 -N.sub.2 -He as the lasing medium. An inherent advantage of the Nd:YAG laser is that its emission is in the order of 1.06 micron wave lengths, where glass is transparent to its laser emission. This charateristic permits the use of a coaxial microscope which uses the same optic elements for both optical viewing and laser focusing. Further, a pulsed Nd:YAG laser is capable of 400 watts of average power, of a pulse frequency of up to 200 pulses per second and of a peak power in excess of 8000 watts for up to 7 milli-seconds. Such high peak power permits the Nd:YAG laser to produce welds of relatively deep penetration, thus insuring the structural security of welded straps of the nuclear fuel rod grids. Such lasers may be operated from a "cold start" with its shutter remaining open, whereby the weld time is determined by the length of time the power is applied to its flash lamps. Such a method of welding is not particularly applicable to a series of relatively rapid welds due to the laser rod warm-up time for each weld in the order of 0.8 seconds. Further, optical path length changes occur until a condition of thermal equilibrium is attained within the laser rod. A second method of operation of the Nd:YAG laser permits the continuous pulse operation of the laser while using its shutter to "pick off" a fixed number of pulses, thus eliminating the effects of laser warm-up and ensuring a uniformity of welds even though a great number of such welds are being effected.
U.S. Pat. No. 3,555,239 of Kerth is an early example of a large body of prior art disclosing automated laser welding apparatus in which the position of the work piece, as well as the welding process, is controlled by a digital computer. Kerth shows the control of laser beams while controlling the work piece as it is moved from side to side along an X axis, horizontally forward and backward along a Y axis and vertically up and down along a Z axis. Typically, pulse drive motors are energized by the digital computer to move the work piece rectilinearly along a selected axis. In addition, the welding is carried out within a controlled atmosphere and, in particular, the pressure and flow of gas into the welding chamber is controlled by the digital computer. Further, a counter is used to count pulses whereby the number of laser pulses applied to the work piece may likewise be controlled.
U.S. Pat. No. 3,803,379 of McKay discusses the problem of maintaining the intensity of a laser beam at precise levels. In particular, this patent notes that when a work piece is changed, it is typically necessary to shut down the laser while a new work piece is being installed and thereafter, to start up the laser bringing it back to a desired level of intensity before resuming machining with its laser beam. In particular, the change of the laser beam intensity will effect corresponding changes in the machining effect on the work piece. To overcome this problem, U.S. Pat. No. 3,803,379 suggests that a diverter mechanism be incorporated along the path of the laser beam, whereby the laser beam may be diverted into a heat sink. Thus, while the work piece is being replaced, the diverter mechanism diverts the laser beam into the heat sink, thus allowing the laser to keep firing at a uniform rate without being shut down so that its temperature, once established under equilibrium conditions, will not be altered between machining operations. Further, experience has shown that with heavy laser usage, the intensity of the laser beam will attenuate with time due to aging of the laser itself as well as of the excitation lamps associated therewith. In addition, the laser beam upon striking a work piece typically throws off gaseous material and debris that may coat the work piece or the laser focusing lens, whereby the machining efficiency is attenuated. Thus it is necessary to periodically calibrate the laser system, whereby the energy level of the laser beam as imparted to the work piece may be accurately controlled. The McKay patent also discloses a relatively simple form of calibration, wherein a portion of the laser beam as directed onto the work piece is diverted by a partially silvered mirror disposed at a 45.degree. angle with respect to the laser beam path, onto a transducer providing an electrical signal indicative of the power of the reflected portion of the laser beam. The transducer is in turn connected to a meter or indicator that provides a visual indication of the power of the reflected beam to permit adjustment or calibration of the strength of the laser beam as directed onto the work piece.
In the initial development of laser machining systems, lasers were employed for individual, low production machining operations. With the development of the art, laser systems were increasingly employed for high production work processing operations as would be controlled automatically by computers. As described above, such high production systems operate efficiently to reposition the work piece, whereby a sequence of welds or other machining operations may be rapidly performed. Under such demands of continuing excitation, laser life becomes a factor in terms of efficient operation and of cost of production. It is contemplated that under high usage where repeated welds are required, as for the production of the above described fuel rod grids, that laser life would be a significant factor to consider. Under heavy usage, the life expectancy of the laser head and, in particular, its excitation lamps would be in the order of several days, and after this life had been expended, it would be necessary to replace at least the lamps, as well as to calibrate the new laser system.
This invention to be described below is particularly directed toward the computer control of laser machining, wherein laser machining is effected in selected of a plurality of modes, each mode varying as to its lasing parameters. For example, in an illustrative embodiment of this invention, a laser source is excited to emit a series of laser pulses onto the work piece in the form of the nuclear fuel rod grid 16 as described above. The fuel rod grid 16 is machined, e.g. welded, with a variety of welds including the corner seam welds 30, the intersect welds 32, the slot and tab welds 34, and the notch seam welds 40. Each such type or mode of machining requires a different set of parameters in terms of the desired power imparted to effect the weld, the pulse rate of and the pulse width of the laser beam. As mentioned above, it is contemplated that the efficiency of the laser and more particularly its excitation lamps will attenuate rapidly with the heavy duty use contemplated by the teachings of this invention. Under such usage, experience has shown that it is necessary to replace the excitation lamps as often as every two days. In actual practice, it has been found necessary to adjust the power level of the laser beam effecting the noted welds at least as often as twice a day to ensure the intregrity of the produced welds. For example, calibration is performed at the beginning of the production day to adjust the laser power, and a second calibration is conducted at the end of the day as a check of the integrity of the welds made during the course of that day. As will be explained in detail below, a laser beam is emitted in each mode or weld type at a programmed power level, which is adjusted or calibrated as a function of the desired lasing parameters for that mode as well as a measurement of the actual power level of the laser beam as directed onto the fuel rod grid.